Sprawling against the trunk of a tree could provide more than half the cooling a koala needs to survive a typical hot summer day.

Koalas don’t have dens to retreat to during a heat wave. Panting and licking their fur may bring some relief, but it uses precious water and raises the risk of dehydration. Flopping against bark that’s cooler than air turns out to be a previously unappreciated part of coping with heat, says ecologist Michael Kearney of the University of Melbourne in Australia.

Natalie Briscoe, also of Melbourne, first noticed that as temperatures soared past 30° Celsius, koalas tended to move lower in trees, stretch out in dishrag poses and even abandon edible eucalyptus trees to loll in inedible Acacia mearnsii trees. Infrared photography and calculations of animal heat loss revealed that the shifts cooled the koalas. Acacia trunks averaged slightly more than 5 degrees Celsius below air temperature, Briscoe, Kearney and their colleagues report June 4 in Biology Letters.

“It gives us a very different perspective on what the habitat requirements are for koalas,” Kearny says. They need trees good at air conditioning as well as trees good for food.

Editor's Note: This article was updated June 20, 2014 to correct the temperature at which koalas' behavior changed.